This invention relates to capacitive transducers, for example for use in displacement and force-responsive devices
Capacitive transducers use various configurations of capacitor electrodes along with springs, diaphragms or other support mechanisms to sense acceleration, force, weight, pressure, displacement or position. Capacitive transducers have much higher output signal than strain gauge transducers, giving them a significant advantage in signal to noise ratio (SNR) and reduced susceptibility to drift from offset voltages in the circuitry, and thermocouple effects in the wiring, but capacitive transducers also generally have a much higher impedance than strain gauge transducers. The higher impedance will degrade the SNR if the transducer drive circuitry is not optimized for the high impedance transducer. Bonin, in U.S. Pat. No. 6,960,945 titled “High-Performance Drive Circuitry For Capacitive Transducers” discusses the drive circuit in detail. The current discussion pertains mainly to aspects of the transducer itself, and only a basic discussion of the circuitry, to aid in understanding the operation of the transducer, will be presented here.
Capacitive transducers operate by sensing the change in capacitance between two or more electrodes caused by a change in position of at least one electrode. By calibrating the readout circuitry accordingly, the output signal may be adjusted to represent the displacement directly, or, in cooperation with springs or other support structure attached to the moving electrode, the output may represent acceleration, force (including weight), or pressure. Capacitive transducers may also generate a force by applying a voltage between two or more electrodes, causing an attractive force between the electrodes proportional to the square of the electric field strength. The force is generally small, on the order of 1 gram for transducers 12 mm in size, but is very useful in certain precision instruments operating with a maximum required force in that range.
Many possible circuits may be used to convert the change in transducer capacitance to a voltage that represents the desired function being measured. The most suitable circuitry for operating capacitive transducers applies an AC carrier or drive signal to one or more drive electrodes, and synchronously demodulates a signal on a pickup electrode to remove the carrier frequency and convert the output to a DC voltage that is representative of the electrode spacing. Ideally the output voltage is a linear function of the electrode spacing or deflection, and is not effected by anything other than the electrode spacing. In practice there are many factors that tend to introduce errors to the output voltage, such as temperature sensitivities in gain and offset voltage, non-linearity, noise, parasitic capacitance, and mechanical imperfections in the transducer. This discussion will be concerned mainly with those aspects directly related to the transducer, with those aspects more directly related to the drive circuitry being discussed in the previously referenced co-pending application. Discussion of the circuitry as required to understand the basic operation of the transducer, as well as the improvements of the current transducer is included.
A prior art capacitive transducer containing three electrodes is described by Bonin in U.S. Pat. No. 5,576,483, “Capacitive Transducer with Electrostatic actuation” issued Nov. 19, 1996. This transducer has a high resolution for measuring displacement and force, but it has several significant problems and limitations that are reduced or eliminated in the transducer of the current invention. FIG. 1 is a simplified exploded view of the prior art transducer. FIG. 2A is a schematic/block diagram of the prior art transducer with the associated electronic circuitry, and FIG. 2B is an electrical schematic representation of the transducer. FIG. 3 is a cross section of the prior art transducer, with the Z-axis scale distorted to show more clearly the thin components of the transducer.
The transducer of the 483 patent includes pickup electrode 29 suspended by springs 30 which are attached to frame 31. Tab 32 is provided for electrical connection to output signal detector/conditioner 37. Pickup electrode assembly 28 consists of previously mentioned pickup electrode 29, springs 30, frame 31, and tab 32. Although referred to as an assembly due to inclusion of a number of parts performing different functions, pickup electrode assembly 28 is generally formed from a single sheet of high strength conductive material, such as a commercially available BeCu alloy. Load button 33 is attached to drive electrode 29, providing for interaction with a sample of material to be tested or measured in some manner.
Lower drive electrode assembly 21L and upper drive electrode assembly 21U are spaced apart from pickup electrode assembly 28 by spacers 27L, 27U. The spacers provide room for pickup electrode 29 to move between lower drive electrode 22L and upper drive electrode 22U. Upper drive electrode 22U is mounted on the bottom surface of assembly 21U in the same manner as lower drive electrode 22L is mounted on the top surface of assembly 21L. Assemblies 21L and 21U are shown in FIG. 1 as mirror images of each other, but they may also be identical. Drive electrode assembly 21L also includes substrate 26L, typically a glass fiber-epoxy composite material commonly used for printed circuit boards, tab 23L for electrical connection to drive electrode 22L, driven shield 24L, and tab 25L for connection to driven shield 24L. Not visible in FIG. 1 are the corresponding tabs and driven shield in upper drive electrode assembly 21U.
The driven shields were required to minimize non-linearity in the prior art transducer due to the large area of frame 31 electrically connected to pickup electrode 29. Having frame 31 mechanically connected to pickup electrode 29 simplified the construction of pickup electrode assembly 28, but also added a large area that would contribute to unwanted parasitic capacitance. Avoiding this parasitic capacitance was done by feeding a unity gain buffered replica of the pickup electrode signal to the driven shields. Because the pickup electrode signal is on frame 31 and the buffered signal is on the driven shields, spacers 27L, 27U must be electrically insulating.
The insulated spacers in the prior art transducer were fabricated by chemically etching the spacers from a sheet of aluminum, and then forming an insulating coating by anodizing the aluminum to form an insulating aluminum oxide coating. This produced spacers having desirable mechanical properties such as relatively low TCE (thermal coefficient of expansion), and high stiffness, but the anodization process was troublesome and sometimes did not produce a satisfactory insulation layer, resulting in reduced yields and even worse, some delayed and intermittent failures of transducers in the field.
The most significant drawbacks of the prior art transducer are due to the electrostatic actuation and position sensing function being shared by the single pickup electrode. Referring to FIG. 2A, note that oscillator 35 and electrostatic controller 36 both apply signals to drive electrodes 22L, 22U. The output of electrostatic controller 36 is a high voltage DC signal between 0 and 600V. Normally only one output of electrostatic controller 36 is active at a time. The high voltage signal is applied to which ever drive electrode it is desired that pickup electrode 29 be attracted towards. Oscillator 35 has two outputs, which are 25 to 50 KHz square waves, switching between ground and 15V. There is a 180 electrical degree phase difference between the two oscillator outputs. The outputs of the electrostatic controller and oscillator are incompatible with each other. If they were connected directly together, neither one would function properly, and the oscillator could be destroyed, as it is only rated for operation at 15V. Resistive buffer B1, and capacitive buffer B2 combine the low frequency high voltage signal from electrostatic controller 36 and oscillator 35, while isolating the controller and oscillator signals from each other. Buffer B1 consists of two resistors typically between 100,000 and 1,000,000 ohms each, and buffer B2 consists of two capacitors typically of 500 to 1000 pF. The resistors must be large enough so that the output capacitance of the electrostatic controller does not load the oscillator, and the capacitors must be large with respect to the capacitance C1 plus C2 of transducer 20B, which is typically about 10 pF. The buffers work well when only very low frequencies (less than 10 Hz) are required from controller 36, and the transducer is operating within its normal travel range.
Unfortunately, the buffers do not work well at higher frequencies, such as are required for such tasks as determining the loss modulus versus frequency of polymers, for which frequencies of at least 100 Hz are desired. At higher frequencies, phase shift and loss in amplitude of the electrostatic controller output signal occurs in the buffers, so that the actual electrostatic signal applied to the transducer drive electrode is different from the electrostatic controller output voltage, creating measurement error.
An even more serious problem is that the transducer cannot be guarantied to always operate within its normal range. If the sample is soft, or the setup is incorrect, so that there is more space than intended between the tip and the sample, the pickup electrode deflection can exceed the maximum stable range which is ⅓ of the nominal electrode spacing. Once the displacement exceeds ⅓ of the nominal spacing, the pickup electrode suddenly and uncontrollably snaps over and contacts the drive electrode. Since this snap over event typically occurs when the output of the electrostatic controller is rather high, at least several hundred volts, possibly up to 600 volts will be stored on one of the capacitors in buffer B2. This voltage is applied to the input of signal detector 37, which is a highly sensitive amplifier with a maximum rated input voltage of 15 to 20V. As the buffer capacitor suddenly discharges into signal detector 37, a large current surge is applied to the oscillator output, as it is pulled below ground. The snap over event frequently destroys either the oscillator or the signal detector or both. The problem was so severe that a software protection scheme was implemented to shut off the electrostatic controller if the deflection of pickup electrode 29 exceeded 5 μm. Although this does protect the transducer, it prevents operation over most of what would be the normal operating range of up to 30 μm displacement.
Another limitation of the prior art transducer is the moving mass of the pickup electrode assembly and tip. This is shown most clearly in FIG. 3. Although the description of the prior art transducer given in the 483 patent indicates that pickup electrode 29 is a single layer of metal foil, the output signal of that design was subject to excessive errors due to its lack of resistance to side forces on probe 33. another prior art patent by Bonin, U.S. Pat. No. 5,553,486 titled “Apparatus For Microindentation Hardness Testing And Surface Imaging Incorporating A Multi-Plate Capacitive Transducer System”, issued Sep. 10, 1996, describes an application where the transducer is used in conjunction with an atomic force microscope to provide both microindentation hardness testing and imaging of the sample surface with the same probe tip, so that even at extremely high magnification, there is no loss of position between indenting and imaging. For this application, prior art transducer 20A shown in FIG. 1 was simply not suitable, due to side to side rocking, and prior art transducer 20C shown in FIG. 3 was developed. The overall width of transducer 20C was about 12.5 mm. The thickness of drive electrode substrates 26U, 26L were about 1.5 mm. Transducer 20C includes grounded shield electrodes 39U, 39L on outer surfaces of substrates 26U, 26L. Instead of single layer pickup electrode assembly 28, transducer 20C has a pickup electrode assembly including top skin 28U spaced apart from bottom skin 28L by core 38. Skins 28U, 28L are identical to pickup electrode assembly 28, except for the added hole for screw 41, which is used to mount probe tip 40. There is a trade off between resistance to side to side rocking and moving mass in the selection of the thickness of core 38. A thicker core increases the resistance to rocking, which is desirable, but also increases the moving mass, which is undesirable. The core is fabricated with many cutouts to reduce the mass, but it still added substantially to the total moving mass. A thickness of 0.635 mm was chosen as a reasonable compromise. With a 75 μm thickness for skins 28U, 28L, the total moving mass with screw 41 was about 220 mg, and probe tip 40 added another 17 mg. Although this mass may seem to be quite small, it is many orders of magnitude greater than the mass of the cantilever probes used in atomic force microscopes. A reduction in mass is desired because lower mass reduces the sensitivity to vibration, and allows more rapid movement of the probe, which is important for imaging in a reasonably short time.
A further problem with the prior art transducer is hysteresis and creep. This was noticeable at larger displacements and loads, and prevented the transducer from operating as an accurate load sensor at more than a few grams. The construction technique of the prior art transducer, where a plurality flat sheets of etched metal layers and insulating substrate layers are bonded together in an assembly, allows a wide range of load and displacement ranges to be created by selecting appropriate spring dimensions and spacer thickness. The problem with the technique is that polymeric materials, such as epoxies or other adhesives used to bond the layers together will begin to deflect and creep excessively at stresses far below those that cause obvious failure. The spring design of the prior art transducer magnified this problem since the most highly stressed region of the springs, where the base of the spring connects to the frame, is adhesively bonded to the spacers. The effect of this on the prior art transducer output is that after a large load is applied to the transducer and then removed, the transducer output signal does not immediately return to zero, but retains some small but undesired offset. Even worse is that the offset is not stable, but gradually disappears over some minutes or hours, so that sensitive measurements made at low loads after a high load measurement will be continuously effected by this changing offset signal.
A final problem with the prior art transducer is that the springs supporting the moving electrodes cannot be made as thin and flexible as desired, due to a propensity to buckle under forces along their length. These forces along the length of the springs are due to TCE mismatches between the various materials used in the construction of the transducer, as well as side loads in the X or Y direction.